Hooley said the patients first responders determined to be critical – or "red" -- had been rushed to hospitals within 15 minutes of the second blast. About three minutes after that, not a single patient was left on Boylston Street where the bomb went off.

By 3:42 p.m., less than an hour after the explosions, a radio announcement notified ambulances that they could return to regular city service.

"A lot of the training we do … fortunately, it kicked in," Hooley said. "When this did happen, everyone knows not just their roles but what their capabilities are. What you can and what you can't do."

For instance, EMS dispatchers couldn't overwhelm the nearby hospitals. They had to evenly spread the patients out, and know which hospitals were able to handle which patients.

The city emergency teams and hospitals studied what emergency responders learned from the 2005 London subway bombing, the 2004 Madrid train bombing and other mass casualty situations.

Biddinger said hospital employees were able to jump into action as soon as the disaster code was announced after the bombing.

"It's something we wouldn't have been able to do 10 years ago," Biddinger said. "The outcomes are a tremendous credit to all the preparation efforts going on, and I think also a tremendous argument for why those efforts need to continue."